As the world's population expands, so does the use of carbon-based fuels with a concomitant increase in the amount of carbon dioxide released into the atmosphere. Most now accept that the ever-increasing cumulative amount of atmospheric carbon dioxide is warming the earth's atmosphere, causing climate change. Within the last twenty-five years, there has been a recognition that the global energy system must move steadily away from a reliance on carbon-rich fuels whose combustion products include the greenhouse gas carbon dioxide. Furthermore, the extraction and movement of fossil fuels around the globe exacerbates global pollution and is a causative factor in the strategic military struggles between nations.
During the latter portion of the 20th century, combustible fuel alternatives, including natural gas and hydrogen, gained prominence as being environmentally cleaner alternatives to higher-carbon based fuels such as oil. This trend towards lower-carbon fuels, combined with the greater energy efficiency of the modern engines has significantly reduced the amount of carbon released into the atmosphere per combustion unit. However, further decreases in carbon released into the atmosphere are necessary to stave off future catastrophes caused by a runaway greenhouse effect.
Hydrogen is the “ultimate fuel.” While the world's oil reserves are depletable, the supply of hydrogen remains virtually unlimited. Hydrogen can be produced from coal, natural gas and other hydrocarbons. Hydrogen can also be produced without the use of fossil fuels, such as by the electrolysis of water using alternative energy sources (e.g., hydroelectric, wind, solar, geothermal, etc.). Furthermore, hydrogen, although presently more expensive than petroleum, is an inherently low cost fuel. Hydrogen has the highest density of energy per unit weight of any chemical fuel and is essentially non-polluting since the main by-product of the oxidation of hydrogen is water. Thus, hydrogen can be a means of solving many of the world's energy related collateral problems, such as climate change, pollution, and a strategic dependency on oil.
To date, the greatest challenge is undoubtedly the need for a cost-effective, on-board hydrogen storage system that will meet the DOE minimum vehicle range of 300 miles within the weight and volume constraints of the vehicle. DOE emphasizes that this is the greatest challenge, since no hydrogen storage technology available today can meet the DOE cost and performance targets even in light of the well-developed hydrogen production and fuel cell technologies (Farrauto, R., ACS Division of Fuel Chemistry, 226th ACS National Meeting, New York, September 2003, paper No. 87, “Catalysts for the Hydrogen Economy”). Although hydrogen can be stored in several ways, e.g., on a solid adsorbent, as a cryogenic liquid, as a compressed gas, or even as a solid chemical hydride, significant barriers must be overcome with each of these methods before the targeted goals can be achieved.
For example, storage of hydrogen as a compressed gas involves the use of large and heavy vessels. In a steel tank of common design at a typical pressure of 136 atmospheres, only about 1% of the total weight is that of the hydrogen gas. This is unacceptable knowing that almost 23 moles of hydrogen gas must be oxidized to release as much energy as the combustion of 1 mole of octane. Storage of hydrogen as a liquid also has disadvantages because liquid hydrogen must be kept extremely cold (below −253° C.) and is highly volatile if spilled. Moreover, liquid hydrogen is energetically expensive to liquefy and maintain in the liquefied state. For example, the losses associated with hydrogen evaporation can be as high as 5% per day. Whether stored as a liquid or gas, hydrogen storage is highly dangerous due to the flammability of the gas.
Various other storage approaches have been tried—adsorption of H2 on inert solids, storing liquid petroleum or methanol followed by reforming to H2, and decomposition of solid hydrides to form H2. Conventional adsorption methods and materials have been shown to be completely inadequate. Although reports in 1997 of high hydrogen adsorption levels on carbon nanotube adsorbents was thought to solve the storage problems (Dillon, A. C., Jones, K M., Bekkedabl, T. A., Kiang, C. H., Bethune, D. S., Heben, M. J., “Storage of Hydrogen in Single-walled Carbon Nanotubes,” Nature (London), 386(6623), 377-379 (1997); Chambers, A., Park, C., Baker, R. T. K., Rodriguez, N. M., “Hydrogen Storage in Graphite Nanofibers,” J. Phys. Chem., 102(22), 42534256 (1998); and U.S. Pat. No. 5,653,951 in the name of Rodriguez, et al. issued Aug. 5, 1997), attempts to recreate the reported work have been disappointing (Dagani, R., “Tempest in a Tiny Tube,” Chem. & Eng. News, Jan. 14, 2002, p. 25, and Tibbetts, G. G., Meisner, G. P., Olk, C. H., “Hydrogen Storage Capacity of Carbon Nanotubes, Filaments and Vapor-Grown Fibers,” Carbon, 39(15), 2291 (2001)). McEnaney reviewed the state of the art in a review paper in 2003 and concluded that numerous claims had been made, but there was little convincing evidence that hydrogen could be adsorbed at the levels required (McEnaney, B., “Go Further with H2,” Chem. in Britain, 39(1), 24 (2003)).
Hydrogen can be stored as a chemically-bonded metal hydride, and much work is underway to demonstrate such technology. This work centers on the use of hydrogen storage alloy materials (see, e.g., U.S. Pat. Nos. 6,746,645, 6,491,866 and 6,193,929 in the name of Ovshinsky et al.). In practice, H2 is physisorbed onto the storage alloy, separates into atomic hydrogen, and bonds with the metal alloy forming metal hydride. To release the hydrogen from the metal hydride, the metal hydride is heated. These technologies, while promising, introduce other challenges, such as poor gravimetric energy density of the fuel (McEnaney, B., “Go Further with H2,” Chem. in Britain, 39(1), 24 (2003)), and the fact that the solid hydrides must be heated to relatively high temperatures in order to release hydrogen.
The military has identified the need for small portable electric power supplies. The U.S. infantryman, for example, has become extremely efficient through the use of high tech devices; e.g. devices which provide him with communication and night vision capabilities. However, these devices require increasing amounts of portable electric power. Currently available battery packs are heavy and unwieldy and function for only a few hours at a time before requiring recharge. Re-charging devices using fuel cells are under development but these require hydrogen fuel, supplied either from a compressed gas cylinder or more usually, by catalytic treatment of a liquid fuel such as methanol. One downside to the use of methanol is that the catalytic treatment process uses some of the produced hydrogen fuel to convert methanol to hydrogen fuel, which is highly inefficient. However, because it is unfavorable to transport heavy, high pressure compressed hydrogen gas cylinders, methanol is still the re-generator of choice.
U.S. Pat. No. 5,518,528 issued May 21, 1996 in the names of Glenn M. Tom and James V. McManus, describes a gas storage and dispensing system, for the storage and dispensing of gases, which comprises an adsorption-desorption apparatus, for storage and dispensing of a gas, e.g., a hydride gas. The gas storage and dispensing vessel of the Tom et al. patent reduces the pressure of stored sorbate gases by reversibly adsorbing them onto a carrier sorbent medium such as a zeolite or activated carbon material.
More specifically, such storage and dispensing system comprises: a storage and dispensing vessel constructed and arranged for holding a solid-phase physical sorbent medium, and for selectively flowing gas into and out of said vessel; a solid-phase physical sorbent medium disposed in said storage and dispensing vessel at an interior gas pressure; a sorbate gas physically adsorbed on the solid-phase physical sorbent medium; and a dispensing assembly coupled in gas flow communication with the storage and dispensing vessel.
The storage and dispensing vessel of the Tom et al. patent thus embodies a substantial advance in the art, relative to the prior art use of high-pressure gas cylinders. Conventional high pressure gas cylinders are susceptible to leakage from damaged or malfunctioning regulator assemblies, as well as to rupture or other unwanted bulk release of gas from the cylinder if internal decomposition of the gas leads to rapid increasing interior gas pressure in the cylinder.
It would therefore be a significant advance in the art of hydrogen storage to provide an improved storage and dispensing apparatus and decomposition method based on the storage and dispensing vessel of Tom et al., which can adsorb substantial quantities of gaseous hydride and can safely and easily be used without risk to the user.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.